The present invention is directed to a semiconductor laser, and a method for making such, having a high thermal conductivity heat sink component enabling the laser to be operated at higher temperatures.
Long-wavelength semiconductor lasers, which emit tunable infrared radiation in the 3 to 30 micron spectral range, are primarily used in tunable diode laser (TDL) spectroscopy systems. TDL spectroscopy, which involves tuning the laser around the absorption bands of a particular molecule, can readily measure sub parts per billion concentrations of trace gases making it a useful tool for detecting and monitoring gaseous pollutants. New pollution emission standards dictated by the Clean Air Act of 1990 will require the monitoring of thousands of smokestacks and other pollution sources throughout the United States. TDL spectroscopy, with its high resolution capabilities, is ideally suited for such monitoring. Atmospheric chemists are presently designing and building TDL spectrometers to measure trace gas concentrations throughout the Earth's atmosphere (M. Loewenstein, "Diode Laser Harmonic Spectroscopy Applied to In Situ Measurements of Atmospheric Trace Molecules", J. Quant. Spectros. Radiat. Transfer, 1988, 40, p. 249). Moreover, long-wavelength tunable diode lasers can be used in feedback control systems to actually reduce emission of pollutants (J. A. Sell, "Tunable Diode Laser of Carbon Monoxide in Engine Exhaust", SPIE, 1983, 438, p.67). Advances in laser performance may eventually allow extension of this pollution control technology to automobiles thus greatly expanding the market for long-wavelength lasers. TDL spectroscopy has also been used to study sub-monolayer concentrations of adsorbates on substrate surfaces (V. M. Bermudez, R. L. Rubinovitz and J. E. Butler, "Study of Vibrational Modes of Subsurface Oxygen on Al (111) Using Diode Laser Infrared Reflection-Absorption Spectroscopy", J. Vac. Sci. Tech., 1988, A6, p. 717). Due to the non-invasive nature of the laser probe, this technique can provide useful information on catalytic reactions and chemical processes (J. E. Butler, N. Bottka, R. S. Sillman, D. K. Gaskill, "In Situ, Real-Time Diagnostics of OMVPE Using IR-Diode Laser Spectroscopy", J. Crystal Growth, 1986, 77, p. 163).
An important TDL spectroscopy feature is its ability to identify and differentiate among compounds that contain different isotopes of a particular element. TDL spectrometers can therefore be used to monitor the motion of isotope-tagged tracer molecules. For example, pollutants tagged with .sup.13 C could be released in the atmosphere and their motion monitored with airborne TDL spectrometers. This offers the unique advantage of observing chemical reaction pathways.
Another example of isotope tracing is in medical diagnostics. Metabolic pathways can be monitored by measuring the .sup.13 CO.sub.2 /.sup.12 CO.sub.2 ratio in the exhaled breath of a patient who has been administered a substance tagged with the non-radioactive isotope .sup.13 C. The .sup.13 CO.sub.2 production correlates directly with the rate at which the particular substance is metabolized. For example, a simple diabetes test would involve feeding a patient .sup.13 C-labeled sugar and monitoring the .sup.13 CO.sub.2 production rate. Such non-invasive analysis of metabolic pathways can form the basis for a whole new field of health research and patient diagnosis. (U. Lachish, S. Rotter, E. Adler, U. El-Hanany, "Tunable Diode Laser Based Spectroscopic System for Ammonia Detection in Human Respiration", Rev. Sci. Instrum., 1987, 58, p. 923, and R. M. Scheck and D. L. Wall, "Medical Diagnostics with TDLs", Photonics Spectra, January 1991, p. 110).
Although some near-infrared spectrometers based upon III-V semiconductor lasers have been developed (D. E. Cooper and R. U. Martinelli, "Near-Infrared Diode Lasers Monitor Molecular Species", Laser Focus World, November 1992, 133), the most widely used TDL spectrometers are based upon narrow bandgap IV-VI semiconductor (also known as lead salt) lasers. Temperature tuned IV-VI semiconductor lasers operate in the 3 to 30 .mu.m spectral region, where gas molecules have their strongest absorption lines, and continue to exhibit better performance characteristics than lasers made from other narrow bandgap semiconductors such as HgCdTe (R. Zucca, M. Zandian, J. M. Arias, and R. V. Gil, "HgCdTe Double Heterostructure Diode Lasers Grown by Molecular Beam Epitaxy", J. Vac. Sci. Technol., 1992, B 10, p. 1587; A. Ravid and A. Zussman, "Laser Action and Photoluminescence in an Indium-Doped n-type Hg.sub.1-x Cd.sub.x Te (x=0.375) Layer Grown by Liquid Phase Epitaxy", J. Appl. Phys., 1993, 73, p. 3979). However, by comparison with research on III-V and II-VI semiconductor materials and devices, research on IV-VI semiconductor materials and devices has lagged. Consequently, techniques that may prove successful in improving the performance of IV-VI semiconductor lasers have yet to be explored.
Maximum operating temperature is presently considered the most important limiting factor for IV-VI semiconductor tunable diode lasers. The highest-known operating temperature for devices operated in continuous wave (cw) mode, which is preferred over pulsed mode for infrared spectroscopy applications, is 203K for a laser emitting in the 3.5 .mu.m range (Z. Feit, D. Kostyk, R. J. Woods, and P. Mak, "Single-Mode Molecular Beam Epitaxy Grown PbEuSeTe/PbTe Buried Heterostructure Diode Lasers for Co.sub.2 High-Resolution Spectroscopy", Appl. Phys. Lett., 1991, 58, p. 343). Longer wavelength devices have even lower maximum operating temperatures. Thus, low operating temperatures necessitate the use of cumbersome liquid nitrogen or liquid helium cooling systems in spectrometers based upon these laser devices. A low laser operating temperature also limits the tuning range of individual devices. If TDL operating temperatures can be increased to above 220 to 230K then thermoelectric cooling (TEC) modules could be used enabling a significant simplification of TDL spectrometer instrumentation. Any increase in maximum operating temperature will also expand TDL tuning range, thus further simplifying TDL spectrometer operation.
Thermal modeling of IV-VI semiconductor lasers (R. Rosman, A. Katzir, P. Norton, K. H. Bachem, and H. Preier, "On the Performance of Selenium Rich Lead-Salt Heterostructure Laser with Remote p-n Junction", IEEE J. Quantum Electronics, 1987, QE-23, p.94) shows that there is a large difference under maximum operating temperature conditions, as much as 60 degrees, between the heat sink temperature and the active region temperature. This thermal gradient is reflected in the maximum operating temperature difference between pulsed and cw operation, observed to be as much as 120 degrees (B. Spanger, U. Schiessl, A. Lambrecht, H. B ottner, and M. Tacke, "Near-Room-Temperature Operation of Pb.sub.1-x Sr.sub.x Se Infrared Diode Lasers Using Molecular Beam Epitaxy Techniques", Appl. Phys. Lett., 1988, 53, p. 2582). Improving heat removal from the active region would therefore lead to an increase in the maximum operating temperatures of IV-VI semiconductor lasers. The major factor limiting heat dissipation from the active region is the substrate which is still attached to the laser structure.
A substrate removal procedure has been developed for III-V semiconductor laser fabrication by E. Yablonovitch, E. Kapon, T. J. Gmitter, C. P. Yun, and R. Bhat, in "Double Heterostructure GaAs/AlGaAs Thin Film Diode Lasers on Glass Substrates", IEEE Photonics Tech. Lett., 1989, 1, p. 41. According to this technique, an AlGaAs/GaAs/AlGaAs laser structure is grown upon a GaAs substrate with a 500 .ANG. AlAs selectively etchable release layer interposed between the substrate and laser structure. The laser structure is then supported from above by Apiezon W.TM. wax ("black wax") while the AlAs layer is etched with dilute HF. This "epitaxial lift-off" (ELO) process, which was developed to enable hybrid device packaging, does not degrade the performance of the laser device. Other examples of methods of epitaxial liftoff are seen in F. Agahi, K. M. Lau, A. Baliga, D. Loeber, and N. Anderson, "Photo-Pumped Strained-Barrier Quantum Well Lasers Fabricated by Epitaxial Liftoff", ISDRS, University of Virginia, Charlottesville, Va. (1993); C. Camperi-Ginestet, M. Hargis, N. Jokerst and M. Allen, "Alignable Epitaxial Liftoff of GaAs Materials with Selective Deposition Using Polyimide Diaphragms", IEEE Transactions Photonic Tech. Lett., Vol. 3, No. 12, pgs. 1123-1126, December 1991; and E. Yablonovitch, E. Kapon, T. J. Gmitter, C. P. Yun and R. Bhat, "Double Heterostructure GaAs/AlGaAs Thin Film Diode Lasers on Glass Substrates, IEEE Photonic Tech. Lett., Vol. 1, No. 2, pgs. 41-42, February 1989.
However, epitaxial-lift off methods using wax are difficult, time-consuming, require extra handling steps and are not readily adaptable to mass production techniques. A method for producing a laser not confined to the limitations of epitaxial lift-off using the wax method and which resulted in the production of a laser operable at higher temperatures would be desirable.